Low cost thermal or electrostatic printers are often of the type having a moving printhead. In such printers the printhead is moved across the paper, defining a rectangular field within which characters can be printed.
The carrier assembly for a moving printhead must hold the printhead in intimate flat contact with the sensitized paper to be printed upon, and must maintain the contact force within narrow margins. It must do this while allowing the head to be moved smoothly and with little friction across the paper along a particular path. In addition, the carrier should be easily removable for servicing the printhead or related parts.
Various designs have been used to meet alignment and contact force criteria. Some designs involve complicated adjustable mechanisms. Some involve mechanisms that do not lend themselves to easy assembly, or disassembly and re-assembly, because, for example, of captive parts accessible only after a substantial disassembly of the mechanism.
If the functions of aligning the printhead, guiding its motion, and generating contact pressure are each performed by separate mechanical constructs the number of parts increases, with a corresponding increase in the number of possible failure modes. It is advantageous if the number of parts can be reduced by using mechanical constructs having multiple functions. The advantage is greater still if the ease of assembly and disassembly can be enhanced at the same time.
Accordingly, a principal object of the present invention is to provide a transport mechanism for a moving head printer wherein the force available form a single spring provides both the contact force of the printhead with the paper, as well as a force for keeping the printhead carrier aligned and engaged with a guide.
Another object of the invention is to provide in a moving head printer a transport mechanism particularly well suited for cooperation with a platen that pivots to maintain intimate flat contact between the printhead and the paper.
A further object of the invention is to provide such a transport mechanism having a carrier that is easily removed without substantial disassembly of the printer.
These and other objects of the invention will become apparent to those skilled in the art as the description of the invention proceeds.
According to a preferred embodiment of the present invention the same force that provides the contact pressure between the printhead and the paper also creates a rotational tendency in the printhead carrier, thereby keeping the carrier engaged with cross slides that support and guide it for motion across the paper.
The paper passes over a platen that supports the paper for contact with the printhead. The platen is approximately as long as the paper is wide, and is at least as wide as the indicia printed during any single pass of the printhead. The lengthwise axis of the platen is generally parallel to the path of printhead motion.
Above the paper and generally parallel to both the surface of the platen and to its lengthwise direction is a first cross slide. The printhead carrier slidably engages the first cross slide in a direction that is away from the platen and toward the first cross slide. Spaced parallel to the first cross slide, and at a distance away from it that is somewhat less than the length of the carrier, is a second cross slide. The carrier slidably engages the second cross slide from a direction opposite that for the first cross slide. Because the carrier engages each cross slide from an opposing direction a suitable force applied to the carrier will keep it slidably engaged with them. A groove in the carrier engages one of the cross slides and simultaneously aligns the carrier upon both cross slides.
An arm is hinged to the carrier at a point between the two cross slides. The axis of the hinge is parallel to the cross slides. The printhead is attached to the arm at a suitable point by any convenient means. When the arm is moved about the hinge and away from the carrier the printhead contacts the platen. The locations of the cross slides and the shapes of the carrier and the arm are such that the printhead lies in, or very nearly in, the plane of the printing surface of the platen when the arm is urged against the platen.
The force from a spring moves the arm away from the carrier and urges it against the platen. The same force also exerts itself as a rotational tendency of the carrier about the hinge axis, which keeps the carrier engaged with the cross slides. A rotational tendency about the hinge axis is ensured by arranging that the extension of a line perpendicular to the arm at the point of contact with the platen does not pass between the two cross slides.
If the extension of such a line does pass between the two cross slides there will be a rotational tendency about each cross slide such that both cross slides must be on the same side of the carrier, instead of on opposing sides.
The printhead cooperates in an advantageous way with the platen which is pivoted about a pivot axis parallel to the hinge axis. In response to the contact force of the printhead against the platen, the latter rotates about the pivot axis to align its printing surface parallel to the printing surface of the printhead. This helps maximize the degree of flat intimate surface contact of the printhead with the paper.
The carrier is easily removed by manually squeezing the arm and carrier together with the thumb and forefinger to override the force of the spring, while rotating the carrier about the first cross slide until the assemblies are clear of each other. The carrier is easily re-installed by reversing the steps.